Upon arriving on Mars, the Viking space probes each dispatched a lander to the surface. In the predawn hours of July 21,1976, at NASA's Jet Propulsion Laboratory in Southern California, scientists and spectators watched anxiously for humankind's first close-up look at the surface of Mars. What the pictures revealed was neither any obvious sign of life, nor a landscape as desolate as that of Venus, but a different kind of world, with a character and history all its own. The landers dug up Martian soil and tested it for signs of biological activity. The tests found some unexpected chemical reactions, but no clear evidence of life. Meanwhile, a pair of Viking orbiters took images that showed dry river beds, indicating that Mars once had liquid water on its surface and, therefore, a denser atmosphere, perhaps one sufficient to support life.

Discovery:In 1609, Galileo Galilei first viewed Mars through a telescope.

Size:Mars is the fourth planet from the Sun and the seventh largest in our solar system.

Orbit:Mars orbits the sun at an average distance of 228,000,000 kilometers
from the Sun. It takes Mars
668.98 Earth days to travel around the Sun.

Comparison to Earth:Of all the planets in the solar system, Mars is the most Earth-like in
terms of its water patterns. Mars has polar ice caps that grow and
recede with the seasons and has evidence of water channels similar to
those on Earth today.

Terrain:Mars is mostly ancient, cratered highlands. The
northern hemisphere of Mars, however, consists of plains. Mars has a
diverse landscape of canyons, mountains, volcanoes, craters and dry river beds.

Interior:Scientists believe that the dense core of Mars comprises 1,700
kilometers in radius; a rocky, molten mantle (denser than the Earth's), a thin crust, and a small core.

Water:As indicated by its dry river beds, Mars was once home to an abundant
supply of water. The height of its water supply was likely around 3.5
billion years ago, when there may have been large lakes and even oceans.
Water supply on Mars seems to have lasted for only a brief time, and
then something went terribly wrong. The atmosphere thinned out, and the
planet's ozone layer, if it had one, collapsed, exposing the surface to
sterilizing solar ultraviolet light. The rivers and streams evaporated
or froze into the ground.

Exploration:Mars has been the destination of numerous space missions. In 1965,
America's Mariner 4 was the first spacecraft to get a close look at
Mars. These early pictures, stored on a 4-track tape recorder, took four
days of transmission to arrive on Earth. Mariners 6 and 7, spacecrafts
in the subsequent mission to Mars, took closer pictures of the surface.
In 1971, Mariner 9 was the first spacecraft to orbit Mars. A few years
later, Vikings 1 and 2 landed on Mars  for an even closer look at
the planet. It wasn't until 1997 that U.S. spacecrafts again landed on
Mars. This time, under the Mars Pathfinder mission, which sent land
rovers to roam the terrain and analyze rocks. Present exploration of
Mars includes two orbiting satellites, a polar cap lander mission, and a
probe mission to learn more about Mars'climate and soil properties.

Atmosphere:Today, Mars has a very thin atmosphere composed of carbon dioxide,
nitrogen, argon and tiny amounts of oxygen and water. The atmosphere is
thick enough to produce a small greenhouse effect and to support strong
winds and dust storms that sometimes grip the planet for several months.

Temperature:The average temperature on Mars is a cold -55 degrees Celsius, or -67 Fahrenheit.

Magnetic fields:Mars has several weak magnetic fields in various parts of the planet, probably left-overs of a global magnetic field from long ago.

Satellites:Mars has two small satellites, Phobos and Deimos, which orbit in close proximity to the planet.